how can i make a robe that follow leg movement without wasting too much memory on collision?
Try Copy Rotation Constraints. Place them on the bone chains of the skirt bones and make them follow the leg bones, adjusting their influence based on how closely you want the skirt to follow the movement.

For extra control, you could create one set of skirt bones to auto-follow the legs and make them the parent of an additional bone for manual control.



The included .blend only has two bone chains for the coat, and they were done with automated weights, but it should guide you right.